The studies described in this report were designed to investigate factors that could influence the behavior of erythrocytes following their interaction with monoclonal antibodies (mAb) in a fully homologous experimental opsonization system in vivo. The clearance profiles and tissue distribution of target erythrocytes were examined in both normal and decomplemented rats preinjected with rat IgG2a or IgG2b mAb directed against the same or different sites on RT1A(a), the classical class I major histocompatibility complex antigen of the DA rats. Complement played a major role in augmenting the clearance and promoting hepatic sequestration of target erythrocytes in rats preinjected with IgG2a mAb directed against the S site. In contrast, an intact complement system was not an essential requirement for erythrocyte clearance when S site-specific IgG2a mAb were used. With each antibody tested, (DA x PVG)F1 cells, expressing about half as much antigen, were removed significantly slower than DA erythrocytes, this finding being more pronounced when the animals had been preinjected with mAb of the IgG2a isotype. A comparison of the tissue distribution of DA and (DA x PVG)F1 erythrocytes indicated that hepatic uptake was greater for target cells expressing higher antigen density. A considerable degree of heterogeneity was observed in the in vivo behavior of the target erythrocytes with three groups of IgG2b mAb that recognized different sites on the class I molecule. The S site-specific IgG2b mAb were much more efficient in the hepatic Fc receptor-mediated clearance system than were the P site-directed mAb of the same subclass. Our results suggest that antibody specificity may also be a contributory factor, in addition to antibody isotype and target cell antigen density, in determining the rate of target cells in vivo.

We describe an investigation of the Fc-receptor dependent function of the hepatic component of the mononuclear phagocyte system (MPS) in rats. Erythrocytes sensitized with the rat IgG2b monoclonal antibody (JY1/98) were cleared by the liver in decomplemented splenectomized rats. This immune clearance was Fc-receptor dependent since it was effectively inhibited by immune complexes of bovine serum albumin (BSA) and rabbit-anti-BSA antibody formed either in vivo or in vitro. Immune complexes formed with F (ab')2 fragment of the rabbit anti-BSA antibody had no effect. Heat-aggregated human gamma globulin was virtually without any competitive activity in the Fc-mediated clearance system. Immune complexes inhibited the hepatic clearance of antibody-sensitized erythrocytes but did not have a significant effect on the early rapid removal of erythrocytes pre-coated with antibody and complement.

The function of the hepatic component of the mononuclear phagocyte system (MPS) was investigated in the rat using N-ethylmaleimide (NEM)-treated erythrocytes and erythrocytes sensitized with a rat IgG2a monoclonal antibody (R2/15S) directed against the rat RT1Aa class I antigen. The clearance of the antibody-sensitized erythrocytes was biphasic, with the initial phase being rapid and complement dependent. This clearance was entirely hepatic and could be reduced or prevented by the infusion of cobra venom factor (CVF), complement fragments or small amounts of immune complexes, prepared either at equivalence or 10-fold antigen excess. NEM-treated cells were removed rapidly from the circulation of normal rats (t 1/2 6.2 +/- 0.4 min) and their clearance rate was not influenced by complement depletion. Most of the cells accumulated in the liver, although a small proportion (17%) was removed by the spleen. Immune complexes formed at equivalence or heat-aggregated human gamma globulin (HAGG) delayed the removal of these cells from the circulation, although significantly larger amounts of HAGG were required to do this. Antigen-excess immune complexes had no effect.

The splenic component of the mononuclear phagocyte system (MPS) was investigated in decomplemented rats by determining the clearance from the blood of erythrocytes coated with a monoclonal antibody (R3/13). The infusion of immune complexes (IC), prepared at 10-fold antigen excess, at an appropriate time during the erythrocyte clearance produced a significant increase in the T1/2 of the antibody coated cells. Immune complexes formed with the F(ab')2 fragment of the rabbit antibody did not have any significant effect. A positive correlation was seen between the dose of immune complex infused and the degree of inhibition of erythrocyte clearance. The influence of red cell antigen number on the behaviour of erythrocytes sensitized with R3/13 was studied by comparing the clearance of DA and (DA X PVG) F1 erythrocytes. F1 erythrocytes, with only half the number of specific antigens on their surface that bind R3/13 antibody were cleared much more slowly (82 +/- 2.6 min, mean +/- s.e.) by the spleen than the DA erythrocytes (44 +/- 1.5 min P less than 0.001). Both cell suspensions were equally susceptible to inhibition by soluble IC. These studies show that the number of specific antigens on the red cell surface influences the rate at which sensitized cells are removed by splenic macrophage Fc receptors but not their susceptibility to inhibition by IC. Our results draw attention to a major defect in the use of autologous erythrocytes coated with anti-rhesus (D) immunoglobulin to assess macrophage Fc receptor function in man.

A patient with rheumatoid arthritis developed severe exacerbation of symptoms 18 hours after an injection of gold thiomalate (sodium aurothiomalate). Immune complexes were present in his serum and synovial fluid; in the synovial fluid they were associated with intense complement activation. The effect of gold salts on splenic reticuloendothelial function was determined by measuring the clearance of heat-damaged erythrocytes from the circulation. Gold thiomalate (50 mg) substantially delayed clearance in the patient but had no effect in four other patients with rheumatoid arthritis who had not had a postinjection reaction. Severely impaired clearance also occurred in three out of four healthy people given 100 mg gold but they remained asymptomatic. The postinjection reaction may be an immune-complex disease that is triggered in certain patients because gold transiently inhibits reticuloendothelial function.

The function of the splenic reticuloendothelial system in patients with rheumatoid arthritis (R.A.) was assessed by determining the clearance of autologous heat-damaged erythrocytes from the circulation. 11/13 patients with active R.A. had a defect in the clearance of red cells and there was a significant inverse correlation between splenic function and the level of circulating immune complexes detected by a Clq-binding assay. Splenic function was normal in 13 patients with inactive disease who had been treated with either gold or prednisone. A serial study on 1 patient with active R.A. showed a clear correlation between fluctuations in disease activity, splenic function, and the level of circulating immune complexes.